The present invention relates to a photosensitive printing plate, suitable for waterless offset printing, which comprises a support, a negative photosensitive layer, and an overlying, printing-ink repellent silicone rubber layer.
Photosensitive printing plates of the type mentioned which contain, as photosensitive compounds, polycondensation products of aromatic diazonium compounds are known from EP-A No. 0,226,201. The photosensitive layer becomes insoluble on exposure and can be developed using organic solvent mixtures. During development, the areas of the silicone rubber layer which are above the soluble, uncured areas of the photosensitive layer are removed together with these soluble areas, although they are themselves not soluble in the developer. However, it is necessary that the developer contains an adequate amount of organic solvents which swell, and thereby soften, the silicone rubber layer.
In U.S. Pat. No. 3,894,873, printing plates are described for waterless offset printing which have a photosensitive layer made from a photopolymerizable mixture. On exposure, the layer polymerizes and crosslinks with the overlying silicone rubber layer. After exposure, the plate is treated with a liquid which swells the silicone rubber. The silicone rubber swells at the unexposed points and can be removed by abrasion, but adheres firmly at the exposed points. Since the photopolymerizable layer is sensitive to atmospheric oxygen and since the silicone rubber layer is extremely oxygen-permeable, an oxygen-impermeable covering film is generally necessary; this is laminated onto the silicone layer and removed after exposure. Together with the silicone layer, this film increases the distance of the photosensitive layer from the image layer of the original and thus reduces the resolution.
Printing plates having photopolymerizable layers of the above-described type are extremely photosensitive as such. However, in the embodiment intended for waterless offset printing, relatively long exposure is necessary, obviously in order to achieve adequate bonding of the printing ink-repellent silicone layer to the photopolymerizable layer. An increase in the photosensitivity of such plates would therefore be desirable.
Printing plates for offset printing by means of fountain solutions are furthermore known which have photosensitive layers made from a diazonium salt polycondensation product, a compound which can be polymerized by means of free radicals, a photopolymerization initiator and at least one binder. Such plates are described in GB-A No. 2,044,788. The better resolution of the copy compared with pure photopolymerizable layers and the increased length of print runs compared with layers which contain only diazonium compounds as photosensitive substances are given here as advantages. In certain cases, it is also possible to carry out development using aqueous solutions or even using pure water.
In EP-A No. 0,167,963, a similar printing plate is described which contains a diazonium salt polycondensation product containing, in addition to diazonium salt units, compounds which are free of diazonium groups and are capable of condensation with formaldehyde.
The last-mentioned printing plates are not intended, and nor are they suitable, for waterless offset printing.